The invention relates generally to fiber laser amplifiers and more particularly to a system and method for switching a high power amplified laser engine between multiple output fibers.
Laser weapons systems have been the subject of much research and development in recent years. These systems commonly employ optical fiber laser amplifiers that amplify light from a single-mode master oscillator laser output by using a pump laser to increase the output power of the system.
A common method to generate a high average power, near diffraction-limited beam for a directed energy (DE) laser weapon is to utilize spectral or coherent beam combining (SBC or CBC) of multiple narrow-linewidth fiber amplifiers in a laser engine. In SBC and CBC, light emitted from a laser engine comprising an array of fiber amplifiers is combined into a single free space beam. For many DE missions, particularly those related to air platform self-defense, it is advantageous to emit beams from multiple beam director turret locations on the platform to provide enhanced field of regard and protect against threats arriving from any angle. Yet to minimize payload size and weight (SaW), it is also advantageous to deploy only a single laser engine on the platform. Hence, there is a need to quickly and efficiently switch laser power from the laser engine between multiple beam director turrets. One technique used at DE-class power levels (typically 100 kW or more) is a free space optical switch, e.g., using one or more moving mirrors positioned downstream of an SBC or CBC beam combiner. This is impractical for numerous reasons.
One of these reasons is that such a system requires routing high power line-of-sight beams through the interior of the platform. Another reason is that large projection distances result in susceptibility to static and dynamic beam drifts and platform-induced jitter. In addition, large free space optics are susceptible to contamination and laser-induced damage. While all these issues are significant, the need for line-of-sight routing imposes the most severe constraint against integration on combat air platforms.
A typical, prior-art high power fiber laser amplifier stage 10 is shown in FIG. 1. It comprises a dual-clad fiber 12, with a small doped core (typically ˜20 um diameter) in which the signal laser light propagates in a single transverse mode, and a larger glass cladding (typically several hundred um diameter) in which relatively low-brightness diode pump light propagates. For a typical approximately 1-2 kW amplifier, the fiber core is seeded with 10 W or more of signal light 14, and the cladding is illuminated with 1-2 kW of pump light from pumps 16. Power is effectively transferred via lasing from pumps 16 to the signal as the two waves co-propagate in gain fiber 12. Typically, diode pump light 16 is combined with signal core light 14 in a pump-signal tapered fiber combiner 18, the output of which is spliced directly to the gain fiber 12 input. This amplifier architecture is mature and is widely used by commercial fiber laser suppliers. This architecture results in a single amplified fiber output 20.
High power fiber-to-fiber switches based on free space optics are widely used in the materials processing industry to switch between workstations. However such switches typically employ multimode fiber sources and multimode fiber targets. This is because the single-micron class tolerances required to switch with high efficiency into a single transverse fiber mode are very difficult to achieve and maintain. For SBC and CBC applications such as a DE laser weapon, however, it is required to use large mode area (LMA) fiber operating in a single transverse fiber mode. With 10 um-class fundamental mode field radius, coupling tolerances to overlap the source light with the target fiber core with <1% coupling loss are on the order of 1 um. Moreover, for <1% coupling loss the wavefront of the transmitted beam must be maintained to λ/60 RMS (root mean square). This is extraordinarily challenging for any optical system and to date has limited development of practical high power single-mode fiber-to-fiber switches based on free space optics. While single-mode fiber-to-fiber switches are commercially available for low power sources, they typically impose insertion losses on the order of approximately 1 dB (20%) or more, which for high power applications is unacceptably high.
A single-laser engine, multi-turret, fiber-switched architecture for DE applications has been disclosed in U.S. Pat. No. 8,488,235, as represented in FIG. 2. This patent discloses a method to scale per-fiber power. Laser engine 22 includes a laser master oscillator source 24 that seeds amplifiers 26, each of which generally corresponds to a fiber laser amplifier 10 of FIG. 1. The outputs of fiber amplifiers 26 are coherently combined into single delivery fibers 30, which then feed into free-space optical beam combiners 32 (based on either SBC or CBC) that are located proximate to each beam director turret. In this prior art, tapered fiber bundle (TFB) fiber splitters 28 are utilized in reverse as a fiber combiner, where each input fiber 27 to a respective TFB is spliced to the output of a fiber amplifier 26. If the amplifier outputs are mutually coherent and properly phase-locked and polarization-locked using servo control methods, then nearly 100% of the input light can be combined into any of the output fibers 30 from a respective TFB. The TFB output can be directed via coherent switching to any of its output fibers by changing the piston phases between the inputs by switch controller 34. Multiple TFBs 28 can be arranged as shown in FIG. 1 to enable a single laser engine 22 to feed multiple SBC or CBC beam turrets 32.
While the architecture of U.S. Pat. No. 8,488,235 appears viable, it is not necessarily optimum for all DE architectures, whether based on SBC or CBC. Some of the drawbacks of U.S. Pat. No. 8,488,235 are as follows:
For example, it can be advantageous to configure a CBC beam combiner as a tiled phased array to enable high speed, all-electronic beam steering for aim point maintenance on target. In such a tiled configuration, it is advantageous to maintain as many laser tiles as practicable to provide the largest possible pointing control range (which scales directly with the number of laser tiles). The architecture of U.S. Pat. No. 8,488,235 results in a reduction of the number of laser tiles by a factor equal to the number of turrets N; e.g. switching between N=3 turrets as shown in FIG. 1 would reduce the number of tiles, and the beam steering range, by 3× compared to the number of fiber amplifiers.
Another drawback of U.S. Pat. No. 8,488,235 is that the power in the delivery fiber is increased by a factor of N over that of any individual amp. This can enhance nonlinear optical impairments (self phase modulation and stimulated Brillouin scattering) that either reduce the combining efficiency (for CBC), degrade beam quality (for SBC), or directly limit the power output. To avoid significant performance limitations from such nonlinear impairments it is typically necessary to limit routing fiber lengths to <10 m, which may be insufficient for some applications, or to limit per-fiber power, which can increase system SaW by requiring more fiber amplifiers.
Another potential drawback of U.S. Pat. No. 8,488,235 is that it requires coherence between the N laser channels that are to be combined and switched. This imposes additional system complexity when used to feed a subsequent set of beam combiner turrets based on SBC, since CBC control electronics must also be included in the laser engine. It also imposes superfluous combining loss in the form of decohered light which will be lost in TFBs 28, but which could have otherwise contributed to the brightness of the SBC output beam.
Another potential drawback of U.S. Pat. No. 8,488,235 is that it requires splicing a TFB to the high power output of each fiber amplifier. While it may be possible to improve TFB losses, to date no high power TFB has been demonstrated with <10% insertion loss, which directly impacts system efficiency (hence driving SaW).
Thus, there is a need for a high power laser engine architecture that overcomes the deficiencies of the prior art. There is a further need for a switched multi-fiber amplifier that is switchable between multiple output turrets with flexible routing, fast switching speeds (<100 ms) to support agile re-targeting against multiple threats, and insensitivity to platform deformations or contamination environment.